This invention relates in general to visual displays for electronic devices and in particular to improved focusing apparatus and techniques for field emission displays.
FIG. 1 is a simplified cross-sectional view of a portion of a field emission display 10 including a faceplate 20 and a baseplate 21, in accordance with the prior art. FIG. 1 is not drawn to scale. The faceplate 20 includes a transparent viewing screen 22, a transparent conductive layer 24 and a cathodoluminescent layer 26. The transparent viewing screen 22 supports the layers 24 and 26, acts as a viewing surface and as a wall for a hermetically sealed package formed between the viewing screen 22 and the baseplate 21. The viewing screen 22 may be formed from glass. The transparent conductive layer 24 may be formed from indium tin oxide. The cathodoluminescent layer 26 may be segmented into localized portions. In a conventional monochrome display 10, each localized portion of the cathodoluminescent layer 26 forms one pixel of the monochrome display 10. Also, in a conventional color display 10, each localized portion of the cathodoluminescent layer 26 forms a green, red or blue sub-pixel of the color display 10. Materials useful as cathodoluminescent materials in the cathodoluminescent layer 26 include Y2O3:Eu (red, phosphor P-56), Y3(Al, Ga)5O12:Tb (green, phosphor P-53) and Y2(SiO5):Ce (blue, phosphor P-47) available from Osram Sylvania of Towanda Pa. or from Nichia of Japan.
The baseplate 21 includes emitters 30 formed on a planar surface of a substrate 32, which may include semiconductor materials. The substrate 32 is coated with a dielectric layer 34. In one embodiment, this is effected by deposition of silicon dioxide via a conventional TEOS process. The dielectric layer 34 is formed to have a thickness that is approximately equal to or just less than a height of the emitters 30. This thickness is on the order of 0.4 microns, although greater or lesser thicknesses may be employed. A conductive extraction grid 38 is formed on the dielectric layer 34. The extraction grid 38 may be formed, for example, as a thin layer of polysilicon. The radius of an opening 40 created in the extraction grid 38, which is also approximately the separation of the extraction grid 38 from the tip of the emitter 30, is about 0.4 microns, although larger or smaller openings 40 may also be employed.
In operation, the extraction grid 38 is biased to a voltage on the order of 100 volts, although higher or lower voltages may be used, while the substrate 32 is maintained at a voltage of about zero volts. Intense electrical fields between the emitter 30 and the extraction grid 38 cause field emission of electrons from the emitter 30 in response to the voltages impressed on the extraction grid 38 and emitter 30.
A larger positive voltage, also known as an anode voltage VA, ranging up to as much as 5,000 volts or more but often 2,500 volts or less, is applied to the faceplate 20 via the transparent conductive layer 24. The electrons emitted from the emitter 30 are accelerated to the faceplate 20 by the anode voltage VA and strike the cathodoluminescent layer 26. This causes light emission in selected areas, i.e., those areas adjacent to where the emitters 30 are emitting electrons, and forms luminous images such as text, pictures and the like.
When the emitters 30 emit electrons, the resultant beam of electrons spreads as the electrons travel from the emitter 30 towards the faceplate 20. When the electron emissions associated with a first localized portion of the cathodoluminescent layer 26 also impact on a second localized portion of the cathodoluminescent layer 26, both the first and second localized portions of the cathodoluminescent layer 26 emit light. As a result, the first pixel or sub-pixel uniquely associated with the first localized portion of the cathodoluminescent layer 26 correctly turns on, and at least a portion of a second pixel or sub-pixel uniquely associated with the second localized portion of the cathodoluminescent layer 26 incorrectly turns on. In a color field emission display 10, this can cause purple light to be emitted from a blue sub-pixel and a red sub-pixel together when only red light from the red sub-pixel was desired. This is problematic because it degrades the image formed on the faceplate 20 of the field emission display 10.
In a monochrome field emission display 10, color distortion does not occur, but the resolution of the image formed on the faceplate 20 is reduced by this spreading of the electron beams from the emitters 30. This is exacerbated in either type of field emission display 10 as the resolution of the field emission display 10 is increased by crowding pixels or sub-pixels more closely together.
A second problem that may occur is that the entire emitted beam of electrons may travel at an angle to the path that they were intended to take, i.e., form a tilted beam of electrons. This may occur because of electrostatic effects involving interactions with other pixels. Alternatively, variations in shapes of tips of the emitters 30 or in extraction grid 38 geometry resulting from normal manufacturing variability may result in some electron beams being tilted relative to others. As a result, more than one pixel may be impacted by an electron beam intended to result in light emission from only a single pixel.
These problems may be referred to as bleedover. The likelihood of bleedover is increased by any misalignment between the localized portions of the cathodoluminescent layer 26 and their associated sets of emitters 30. additionally, as the current from any one of the emitters 30 is increased, the problem of bleedover increases.
In some applications, a small field emission display 10 is intended to be viewed through magnifying optics, such as lenses or magnifying reflectors. These applications require a high resolution field emission display 10. High resolution field emission displays 10 use fewer emitters 30 per pixel or sub-pixel. This arises for several reasons, one of which is that a smaller pixel or sub-pixel subtends a smaller area in which the emitters 30 can be provided. As a result, each emitter 30 in a high resolution field emission display 10 has a greater influence on the light emitted from the pixel or sub-pixel associated with it. This increases the need to be able to control electron emissions and the spread of electron emissions from each emitter 30.
In conventional field emission displays 10, attempts have been made to alleviate bleedover in several ways. The anode voltage VA applied to the transparent conductive layer 24 of the conventional field emission display 10 is a relatively high voltage, such as 1,000 volts or more, so that the electrons emitted from the emitters 30 are strongly accelerated to the faceplate 20. As a result, the electron emissions spread out less as they travel from the emitters 30 to the faceplate 20. The gap between the faceplate 20 and the baseplate 21 of the conventional field emission display 10 is relatively small (ca. one thousandth of an inch or twenty-five microns per 100 volts of anode voltage VA), again reducing opportunity for spreading of the emitted electrons.
Some solutions that have been tried for reducing bleedover either increase the anode voltage VA applied to the transparent conductive layer 24 or decrease the spacing between the faceplate 20 and the baseplate 21 in order to reduce spreading of the electron emissions. However, it has been found that these are impractical solutions because the anode voltage VA applied between the transparent conductive layer 24 and the baseplate 21 may cause arcing when either of these solutions is attempted.
Another way in which bleedover is reduced in conventional field emission displays 10 is by spacing the localized portions of the cathodoluminescent layer 26 relatively far apart. This is possible because of the relatively low display resolution provided by conventional field emission displays 10. As a result, the electron emissions impact the correct localized portion of the cathodoluminescent layer 26. However, as the resolution of images displayed by field emission displays 10 increases, the localized portions of the cathodoluminescent layer 26 are necessarily crowded closer together. As a result, bleedover may occur.
One solution that has been employed in conventional cathode ray tubes is to metalize the back surface of the cathodoluminescent layer 26. However, in field emission displays 10, this technique would require an increase of several hundred percent in the anode voltage VA in order to achieve the same luminosity. However, an increase of anode voltage VA in field emission displays 10 requires an increased separation between the faceplate 20 and the baseplate 21. As a result, the electron beam from each emitter 30 spreads out even more in traveling from the emitter 30 to the faceplate 20. Additionally, the increased anode voltage VA itself is objectionable from the perspectives of power consumption and circuit complexity.
One approach to controlling the spatial spread of electrons emitted from a group of the emitters 30 is to surround the area emitting the electrons with a focusing electrode (not shown). This allows increased control over the spatial distribution of the emitted electrons via control of the voltage applied to the focusing electrode, which in turn provides increased resolution for the resulting image. One such approach, where each focusing element serves many emitters, is described in U.S. Pat. No. 5,528,103, entitled xe2x80x9cField Emitter With Focusing Ridges Situated To Sides Of Gate,xe2x80x9d issued to Spindt et al.
Disadvantages to the prior art approaches include the need for another voltage source for the focusing electrode and problems due to variations in turn-on voltage from one emitter 30 to another. When a group of emitters 30 are all affected by a single focusing electrode, some of the emitters 30 may exhibit a turn-on voltage that differs from that exhibited by other emitters 30. The effect that the focusing electrode has on the electrons emitted from each of these emitters 30 will differ. Additionally, some of the current through the emitters 30 will be collected by the focusing electrode. This complicates the relationship between the current through the emitter 30 and the amount of light that is generated at the faceplate 20 because some of the current through the emitter 30 is diverted en route to the faceplate 20 by the focusing electrode. Further, the effects of the focusing electrode may be different for emitters 30 that are closer to the focusing electrode than for emitters 30 that are farther away from the focusing electrode. The lack of control over the amount of light emitted in response to a known emitter current results in poorer imaging characteristics for the display 10.
In magnified, high resolution field emission displays 10, each pixel must be able to provide higher light output because the intensity of the illumination when it reaches the eye of the viewer is reduced in proportion to the magnification needed in order to view it. As a result, the current density in each pixel is increased relative to larger field emission displays 10. As discussed in xe2x80x9cResistivity Effect of ZnGa2O4:Mn Phosphor Screen on Cathodoluminescence Characteristics of Field Emission Displayxe2x80x9d by S. S. Kim et al., J. Vac. Sci. Technol. B 16(4), July, August 1998, resistance in the cathodoluminescent layer 26 itself can significantly affect luminance through several mechanisms, as is explained below in more detail.
A first mechanism is due to a voltage drop occurring in the cathodoluminescent layer 26. Most cathodoluminescent materials are formed from metal oxides or sulfides having resistivities xcfx81 on the order of 1010 xcexa9-cm. An exception is ZnO:Zn, which has a resistivity on the order of 106 xcexa9-cm, but which is poorly suited for use in color field emission displays 10. The materials used to form the cathodoluminescent layer 26 typically are powdered and have particle sizes on the order of two microns or less. In order to provide a reasonably uniform cathodoluminescent layer 26, it is necessary to deposit a cathodoluminescent layer 26 that is three or more particles thick, or six to ten microns thick.
Electrons incident on the cathodoluminescent layer 26 typically only excite fifteen to thirty Angstroms of that portion of the cathodoluminescent layer 26 that is closest to the emitters 30. Although the cathodoluminescent layer 26 is formed on the transparent conductive layer 24, which is typically indium tin oxide having a sheet resistivity of about 25 xcexa9/xe2x96xa1, the voltage drop through the cathodoluminescent layer 26 can amount to a significant percentage of the anode voltage VA applied to the transparent conductive layer 24. In some experiments using low anode voltages VA in vacuum fluorescent displays, the anode voltage VA is reduced by as much as seventy percent or more from one side of the cathodoluminescent layer 26 to the other, thereby reducing the electron-attracting effect of the anode voltage VA substantially. As a result, the number of electrons arriving in the pixel per unit time is reduced, reducing pixel luminosity.
A second mechanism in which the resistance of the cathodoluminescent layer 26 affects pixel luminosity involves localized heating of the cathodoluminescent layer 26 due to the increased current through the cathodoluminescent layer 26. The localized heating reduces the efficiency of the cathodoluminescent layer 26. This phenomenon is known as xe2x80x9cthermal quenchingxe2x80x9d of the cathodoluminescent materials making up the cathodoluminescent layer 26. As a result, the luminosity per incident electron decreases, providing a darker pixel than is needed. Useful lifetime of the cathodoluminescent layer 26, and hence of the display 10 incorporating the cathodoluminescent layer 26, may also be reduced.
All of these effects tend to degrade linearity of the relationship between current through the emitter 30 and luminosity of the pixel associated with the emitter 30. A linear relationship between these two quantities greatly simplifies useful and effective operation of field emission displays 10.
There is therefore a need for a way to increase the linearity of the relationship between pixel luminosity and emitter current to provide robust field emission displays, and especially high resolution field emission displays, without significantly increasing fabrication complexity for such displays.
In accordance with one aspect of the invention, a field emission display includes a faceplate having a transparent viewing layer, a transparent conductive layer formed on the transparent viewing layer and a grille of light-absorbing, opaque insulating material formed on the transparent conductive layer and defining openings within the grille. The light absorption and opacity of the grille increases the contrast of the faceplate. The faceplate also includes a plurality of pixels formed of cathodoluminescent material. Each pixel is formed in one of the openings. The cathodoluminescent material includes a noncathodoluminescent material providing reduced resistivity in the cathodoluminescent material.
Significantly, the light-absorbing, opaque insulating material charges electrostatically in direct response to bleedover of electrons from any one pixel or sub-pixel. As a result, localized electrostatic fields provide enhanced focusing performance together with reduced circuit complexity compared to prior art approaches. Additionally, the noncathodoluminescent material results in more accurate control of voltages accelerating electrons towards the cathodoluminescent material. This, in turn, results in superior display performance, especially for high resolution field emission displays.